Development of a Kinetic ELISA and Reactive B Cell Frequency Assay to Detect Respiratory Syncytial Virus Pre-Fusion F Protein-Specific Immune Responses in Infants

BACKGROUND

Respiratory syncytial virus (RSV) is a major cause of respiratory disease in infants, making vaccination an attractive preventive strategy. Due to earlier reports of vaccine-enhanced disease in RSV-naive children, assessing prior RSV infection is critical for determining eligibility for future infant vaccine trials. However, this is complicated by the presence of maternally transferred maternal antibodies. We sought to develop assays that measure immune responses to RSV pre-fusion (F) protein that discriminates between maternal and infant responses.

METHODS

We measured RSV-specific responses in two groups of children <3 years of age; those with laboratory-confirmed RSV (RSV-infected) and those enrolled prior to their first RSV season (RSV-uninfected). Serial blood samples were obtained and recent infections with RSV and other respiratory viruses were assessed during follow-up. An RSV pre-F-specific kinetic enzyme-linked immunosorbent assay (kELISA) and an F-specific reactive B cell frequency (RBF) assay were developed.

RESULTS

One hundred two young children were enrolled between July 2015 and April 2017; 74 were in the RSV-uninfected group and 28 were in the RSV-infected group. Participants were asked to provide sequential blood samples over time, but only 53 participants in the RSV-uninfected group and 22 participants in the RSV-infected groups provided multiple samples. In the RSV-infected group, most had positive kELISA and RBF during the study. In the RSV-uninfected group, two patterns emerged: declining kELISA values without reactive B cells, due to maternal transplacental antibody transfer, and persistently positive kELISA with reactive B cells, due to asymptomatic undiagnosed RSV infection.

CONCLUSIONS

A kELISA targeting RSV pre-F epitopes and an RBF assay targeting RSV F-specific B cells generally allow discrimination between maternally and infant-derived antibodies.

Structural and Immunoreactivity Properties of the SARS-CoV-2 Spike Protein upon the Development of an Inactivated Vaccine

Inactivated vaccines are promising tools for tackling the COVID-19 pandemic. We applied several protocols for SARS-CoV-2 inactivation (by β-propiolactone, formaldehyde, and UV radiation) and examined the morphology of viral spikes, protein composition of the preparations, and their immunoreactivity in ELISA using two panels of sera collected from convalescents and people vaccinated by Sputnik V. Transmission electron microscopy (TEM) allowed us to distinguish wider flail-like spikes (supposedly the S-protein's pre-fusion conformation) from narrower needle-like ones (the post-fusion state). While the flails were present in all preparations studied, the needles were highly abundant in the β-propiolactone-inactivated samples only. Structural proteins S, N, and M of SARS-CoV-2 were detected via mass spectrometry. Formaldehyde and UV-inactivated samples demonstrated the highest affinity/immunoreactivity against the convalescent sera, while β-propiolactone (1:2000, 36 h) and UV-inactivated ones were more active against the sera of people vaccinated with Sputnik V. A higher concentration of β-propiolactone (1:1000, 2 h) led to a loss of antigenic affinity for both serum panels. Thus, although we did not analyze native SARS-CoV-2 for biosafety reasons, our comparative approach helped to exclude some destructive inactivation conditions and select suitable variants for future animal research. We believe that TEM is a valuable tool for inactivated COVID-19 vaccine quality control during the downstream manufacturing process.

Single-Shot Vaccines against Bovine Respiratory Syncytial Virus (BRSV): Comparative Evaluation of Long-Term Protection after Immunization in the Presence of BRSV-Specific Maternal Antibodies

The induction of long-lasting clinical and virological protection is needed for a successful vaccination program against the bovine respiratory syncytial virus (BRSV). In this study, calves with BRSV-specific maternally derived antibodies were vaccinated once, either with (i) a BRSV pre-fusion protein (PreF) and Montanide^TM ISA61 VG (ISA61, n = 6), (ii) BRSV lacking the SH gene (ΔSHrBRSV, n = 6), (iii) a commercial vaccine (CV, n = 6), or were injected with ISA61 alone (n = 6). All calves were challenged with BRSV 92 days later and were euthanized 13 days post-infection. Based on clinical, pathological, and proteomic data, all vaccines appeared safe. Compared to the controls, PreF induced the most significant clinical and virological protection post-challenge, followed by ΔSHrBRSV and CV, whereas the protection of PreF-vaccinated calves was correlated with BRSV-specific serum immunoglobulin (Ig)G antibody responses 84 days post-vaccination, and the IgG antibody titers of ΔSHrBRSV- and CV-vaccinated calves did not differ from the controls on this day. Nevertheless, strong anamnestic BRSV- and PreF-specific IgG responses occurred in calves vaccinated with either of the vaccines, following a BRSV challenge. In conclusion, PreF and ΔSHrBRSV are two efficient one-shot candidate vaccines. By inducing a protection for at least three months, they could potentially improve the control of BRSV in calves.

Development of a Stable Respiratory Syncytial Virus Pre-Fusion Protein Powder Suitable for a Core-Shell Implant with a Delayed Release in Mice: A Proof of Concept Study

Currently, there is an increasing interest to apply pre-fusion (pre-F) protein of respiratory syncytial virus (RSV) as antigen for the development of a subunit vaccine. A pre-F-containing powder would increase the flexibility regarding the route of administration. For instance, a pre-F-containing powder could be incorporated into a single-injection system releasing a primer, and after a lag time, a booster. The most challenging aspect, obtaining the booster after a lag time, may be achieved by incorporating the powder into a core encapsulated by a nonporous poly(dl-lactic-co-glycolic acid) (PLGA) shell. We intended to develop a stable freeze-dried pre-F-containing powder. Furthermore, we investigated whether incorporation of this powder into the core-shell implant was feasible and whether this system would induce a delayed RSV virus-neutralizing antibody (VNA) response in mice. The developed pre-F-containing powder, consisting of pre-F in a matrix of inulin, HEPES, sodium chloride, and Tween 80, was stable during freeze-drying and storage for at least 28 days at 60 °C. Incorporation of this powder into the core-shell implant was feasible and the core-shell production process did not affect the stability of pre-F. An in vitro release study showed that pre-F was incompletely released from the core-shell implant after a lag time of 4 weeks. The incomplete release may be the result of pre-F instability within the core-shell implant during the lag time and requires further research. Mice subcutaneously immunized with a pre-F-containing core-shell implant showed a delayed RSV VNA response that corresponded with pre-F release from the core-shell implant after a lag time of approximately 4 weeks. Moreover, pre-F-containing core-shell implants were able to boost RSV VNA titers of primed mice after a lag time of 4 weeks. These findings could contribute to the development of a single-injection pre-F-based vaccine containing a primer and a booster.

Respiratory syncytial virus subunit vaccines based on the viral envelope glycoproteins intended for pregnant women and the elderly

Introduction: Respiratory syncytial virus (RSV) causes high morbidity and mortality rates among infants, young children, and the elderly worldwide. Unfortunately, a safe and effective vaccine is still unavailable. In 1966, a formalin-inactivated RSV vaccine failed and resulted in the death of two young children. This failure shifted research toward the development of subunit-based vaccines for pregnant women (to passively vaccinate infants) and the elderly. Among these subunit-based vaccines, the viral envelope glycoproteins show great potential as antigens. Areas covered: In this review, progress in the development of safe and effective subunit RSV vaccines based on the viral envelope glycoproteins and intended for pregnant women and the elderly, are reviewed and discussed. Studies published in the period 2012-2018 were included. Expert opinion: Researchers are close to bringing safe and effective subunit-based RSV vaccines to the market using the viral envelope glycoproteins as antigens. However, it remains a major challenge to elicit protective immunity, with a formulation that has sufficient (storage) stability. These issues may be overcome by using the RSV fusion protein in its pre-fusion conformation, and by formulating this protein as a dry powder. It may further be convenient to administer this powder via the pulmonary route.